This invention relates to steel alloys, commonly designated as specialty steels, and more particularly to steel alloy systems and methods for improving the mechanical properties of alloy steels, reducing the complexity of alloy steel compositions and reducing costs.
The mechanical properties of alloy steels vary with the properties of their free metal boundaries, grain bodies and grain and phase boundaries. Current practices rely on many alloying systems and thermomechanical treatments, such as rolling, pressing, hammering and forging and various chemical and heat treatments to alter the mechanical properties of alloy steels. Current alloying systems are based on the idea of steel microstructure modifications and do not consider the effects of grain boundaries between crystals and alloy phase components on mechanical properties.
Iron (Fe), carbon (C), manganese (Mn), phosphorus (P), sulfur (S), silicon (Si), and traces of oxygen (O), nitrogen (N), and aluminum (Al) are always present in steel, together with alloying elements, such as nickel (Ni), chromium (Cr), copper (Cu), molybdenum (Mo), tungsten (W), cobalt (Co) and vanadium (V). Current alloying systems, steel making and heat treatment practices often procure non-equilibrium segregations of traditionally harmful admixtures (S, P, Sn, etc.), as well as embrittling non-metallic phases on free metal surfaces, grain and phase boundaries during tempering. Chemical heat treatments, such as nitro-carburizing and nitriding cause brittleness and distortion of grain bodies due to formation of a second, large volume phase along grain boundaries, having a harmful effect on the viscous characteristics of steel. For example, the impact strength of steel containing (by weight) 0.25% C; 1.6% Cr; 1.5% Ni; 1.0% W; and 0.6% Mo, is reduced to 2-3 J/cm2, following oil quenching at 980xc2x0 C. and a 24 hour tempering at 500xc2x0 C. (so-called false nitriding).
Another aspect of current steel alloying, making and heat treatment practices is that increases in strength decrease ductility, and in the alternative, increases in ductility decrease strength. Heretofore, no satisfactory compromise has been found between strength and ductility of alloy steels.
Current practices require large numbers of classes and grades of alloy steels, large investments and large inventories to support the requirements of industrial and consumer products. More than 320 grades of specialty steels are produced in the United States; 70-100 in Germany; 140-160 in Great Britain; 60-70 in Sweden; 140-160 in France; 100-120 in Japan; and 140-150 in Russia.
The following alloying systems are typical of current practices:
A: Structural, heat-treatable, carburizing, nitro-carburizing, and nitriding steels 1.
1. Fexe2x80x94Cxe2x80x94Cr
2. Fexe2x80x94Cxe2x80x94Crxe2x80x94Moxe2x80x94Al
3. Fexe2x80x94Cxe2x80x94Crxe2x80x94Nixe2x80x94Mo
B. Die, spring, maraging, and duplex steels
1. Fexe2x80x94Cxe2x80x94Crxe2x80x94Si
2. Fexe2x80x94Cxe2x80x94Crxe2x80x94Sixe2x80x94Vxe2x80x94B
3. Fexe2x80x94Cxe2x80x94Crxe2x80x94Sixe2x80x94Nixe2x80x94Moxe2x80x94(V, Ti)xe2x80x94N
C. High speed tool steels
1. Fexe2x80x94Cxe2x80x94Crxe2x80x94Wxe2x80x94Moxe2x80x94Vxe2x80x94Co.
D. High temperature steels
1. Fexe2x80x94Cxe2x80x94Crxe2x80x94Nixe2x80x94Moxe2x80x94Sixe2x80x94(V, Ti, Nb)
E. Free-cutting steels
1. Fexe2x80x94Cxe2x80x94Crxe2x80x94(Ca, Pb, Se, Te, Sb)
Another aspect of the current practice is that vast, complex facilities are required to support the many current alloying systems. Large sums of money are required to establish and maintain large inventories and complex facilities.
One benefit of the present invention is that strength of steels can be increased without significant reductions in ductility, or in the alternative, ductility can be increased without significant reductions in strength. Another major benefit is that the number of grades of specialty steels for meeting industrial and consumer requirements can be substantially reduced. Still another benefit is that number and complexity of steel making facilities can be substantially reduced. Yet another benefit is that substantial savings can be made in reducing inventories. One more benefit is that various grades of steel can be produced by using a continuous-casting furnace, varying the amount of carbon during melting; better commonality can be achieved for all subsequent metallurgical conversion processes (casting, heating, rolling, heat treatment). Still yet another benefit is that the use of expensive alloying elements, such as nickel (Ni), molybdenum (Mo), titanium (Ti), cobalt (Co), boron (B), and tungsten (W) can be eliminated, except for maraging steels.
The invention resides in the ability of certain combinations of carbon-subgroup surfactants and d-transition metals, which will be described in proper sequence, in xcex1 and (xcex1+xcex3) steels to: 1) modify and control diffusion mechanisms of interstitial elements; 2) reduce or prevent the formation of non-equilibrium segregations of harmful admixtures and brittle phases being formed on free metal surfaces, grain and phase boundaries; 3) alter and control the phase transformation kinetics in steel during heating and cooling.
In a first embodiment of the invention, combinations of silicon, copper and vanadium comprise the carbon-subgroup surfactants and d-transition metals. In a second aspect of the invention combinations of germanium, copper and vanadium comprise the carbon-subgroup surfactants and d-transition metals.
Further aspects, benefits and features of the invention will become apparent from the ensuring detailed description of the invention. The best mode, which is contemplated in practicing the invention, together with the manner of using the invention, are disclosed, and the property, in which exclusive rights are claimed, is set forth in each of a series of numbered claims at the conclusion of the detailed description.